In response to the prevalence of a laptop personal computer, a handheld information terminal such as a personal digital assistant (PDA), and further a cellular phone terminal, mobile radio communication has been used in various places and fields. The mobile radio communication offers numerous advantages to a user who can move while continuing communication, and uses a mobile radio communication terminal device (referred to also as a mobile terminal or a terminal).
Particularly in the field of an intelligent transport system (ITS), a radio base station (referred to also as a base station) placed on a road and a terminal mounted on each vehicle mutually exchange information on surroundings therebetween. At this time, in an advanced safety vehicle (ASV) that prevents a conceivable accident from occurring, it is necessary to implement both of communication performed under the control performed by the base station (infrastructure mode communication) and communication autonomously controlled by each terminal (ad hoc mode communication).
In that regard, in terms of effective use of a radio resource, ease of configuration of a vehicle-mounted terminal, and cost, it is desirable that the same radio resource can be used in the infrastructure mode communication and in the ad hoc mode communication.
However, when each terminal performs the ad hoc mode communication in an autonomous distributed manner using the same radio resource in an area where the infrastructure mode communication under the control of the base station is possible (infrastructure mode communication area), a problem occurs in which a terminal of the transmission partner cannot receive a radio wave due to interference between radio waves of both communications, and a case where the radio resource cannot be efficiently used may occur.
In addition, when a radio wave is emitted using the ad hoc mode communication from a terminal which is out of reach of a radio wave from the base station and uncontrollable, and which has come into a place where the radio wave reaches a terminal in the area of the base station irrespective of scheduling performed by the base station, there arises a problem of a so-called hidden terminal that mutual interference between the radio wave emitted from the terminal and the radio wave emitted from the base station prevents communication to a terminal in the base station area. Therefore, it is still impossible to circumvent the problem that the use efficiency of a radio resource in the base station area is thereby reduced.
As a technology for solving the problems mentioned above, there is a proposed technology which prevents radio wave interference by a terminal in a base station area with infrastructure mode communication. As described in Japanese Patent Application No. 2006-188270 (filed by the same applicant on Jul. 7, 2006), a base station rather intensely emits a control signal in advance to a range having the fear of interfering with an area under the control of the base station (infrastructure mode communication area), thereby advertising information on a radio resource used by the base station. A terminal outside the infrastructure mode communication area, which has received the radio resource information, performs the ad hoc mode communication using a radio resource other than the radio resource used by the base station.
In addition, as described in Japanese Patent Application No. 2006-188271 (filed by the same applicant on Jul. 7, 2006), there is proposed a technology in which, when a terminal in a base station area emits terminal-to-terminal communication information to a terminal outside the area, information on a radio resource used by the base station is added thereto, thereby preventing a terminal that has received the information from using the radio resource used by the base station for ad hoc mode communication.
As with the former one of the prior art technologies described above, in the case where a base station rather intensely transmits control information to an interfering area, even when a terminal of infrastructure mode communication does not exist in a base station area within a range in the reach of a radio wave from an in-coming terminal, and does not interrupt the infrastructure mode communication due to interference, a terminal of ad hoc mode communication outside the area cannot use a radio resource used by the base station, which excessively limits the ad hoc mode communication, and interrupts effective use of the radio resource.
Moreover, ad hoc mode communication performed by a vehicle at a point where the control information from the base station does not arrive under the influence of a shielding object lying midway in a communication path or the like may affect reception performed by a terminal in the base station area (problem of a hidden terminal). When the intensity of the radio wave of the control information from the base station is further increased in order to avoid the phenomenon, the control information reaches even a range which does not interfere with the interior of the base station area in a direction in which a shielding object does not exist. Thus, the use of the radio resource with respect to a terminal which performs the ad hoc mode communication is still excessively limited.
The following is a related art to the invention.    [Patent document 1] Japanese Patent Laid-Open Publication No. JP 2003-249939